To be told you should keep your head up is to say that you should be alert; in which case, you shouldn't be in a position to find yourself ducking *IF* you pay heed to it. 'Heads up' also means to not take your eyes off what it is you are doing; same thing, if you are paying attention to what you are doing you still shouldn't need to be ducking because you can see what's ahead of you<br />
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~F~

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"Duck" still leaves you as a defenseless target, unsure if anything is coming at you or not. "Heads up" means take a look and see that something is coming at you, and choose your defensive action accordingly.<br />
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Yeah, I took the question too literally.

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